Designing object-oriented software is difficult, and structuring a reusable object-oriented software is even harder. Mainly while you are designing an object-oriented software you have to keep in mind that your design should be specific to the problem at hand but also general enough to address future problems and requirements. You also want to avoid redesign, or at least minimize it. Experienced object-oriented designers will tell you that a reusable and flexible design is difficult if not impossible to get “right” the first time. Before a design is finished, they usually try to reuse it several times, modifying it each time.

What is Design Pattern in Object Oriented Design?

Christopher Alexander says, “Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice

“Christopher Alexander, Sara Ishikawa, Murray Silverstein, Max Jacobson, Ingrid Fiksdahl-King, and Shlomo Angel. A Pattern Language. Oxford University Press, New York, 1977.

Christopher Alexander was talking about patterns in buildings and towns, but it’s true for object-oriented design patterns as well.

In general, a pattern has four essential elements:

  1. Pattern name: It’s a handle to describe a design problem, solutions and consequences.
  2. Problem: It describes when to apply the pattern in which circumstances.
  3. Solution: It describes the elements that make up the design, their relationships, responsibilities, and collaborations. It doesn’t describe particular concrete design but a concept or a template that can be applied in various situations.
  4. Consequences: It describes the result and trade-offs of applying the pattern.

There are mainly 3 types of object-oriented design patterns:

  1. Creational: It is mainly dealing with object creation mechanisms, trying to create object in a manner suitable to the situation. The basic form of object creation could result in design problems or in added complexity to the design. (Ref: Wikipedia)
  2. Structural: It ease the design by identifying a simple way to realize relationships among entities. (Ref: Wikipedia)
  3. Behavioral: It identify common communication patterns among objects and realize these patterns. By doing so, these patterns increase flexibility in carrying out this communication. (Ref: Wikipedia)

Here are the list of known design patterns:

CREATIONAL PATTERNS

  1. Abstract factory: Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
  2. Builder: Separate the construction of a complex object from its representation so that the same construction process can create different representations.
  3. Factory method: Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory method lets a class defer instantiation to subclasses.
  4. Prototype: Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
  5. Singleton: Ensure a class only has one instance, and provide a global point of access to it.

STRUCTURAL PATTERNS

  1. Adapter: Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn’t otherwise because of incompatible interfaces.
  2. Bridge: Decouple an abstraction from its implementation so that the two can vary independently.
  3. Composite: Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
  4. Decorator: Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
  5. Facade: Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.
  6. Flyweight: Use sharing to support large numbers of fine-grained objects efficiently.
  7. Proxy: Provide a surrogate or placeholder for another object to control access to it.

BEHAVIORAL PATTERNS

  1. Chain of Responsibility: Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.
  2. Command: Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
  3. Interpreter: Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
  4. Iterator: Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
  5. Mediator: Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.
  6. Memento: Without violating encapsulation, capture and externalize an object’s internal state so that the object can be restored to this state later.
  7. Observer: Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
  8. State: Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
  9. Strategy: Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
  10. Template Method: Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure.
  11. Visitor: Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.